In recent years, a small, light and thin thermal printer has been desired from the market for such use as described in the above, and various types have been proposed.
FIG. 35 is a perspective view showing the structure of a conventional thermal line printer. FIG. 36 is a cross sectional view showing the directions of feeding and ejecting the recording paper in the conventional printer. FIG. 37 is a perspective view showing the whole structure of a handy terminal as an example in which the conventional thermal line printer is installed.
In FIG. 35 and FIG. 36, a recording paper feeding guide 101a is disposed in a body chassis 101, a platen roller 102 having a cylindrical shape is rotatably supported by the body chassis 101, a motor 103 rotates the platen roller 102 through the power transmission of a row of gears 104a, 104b, 104c and 104d, a row of heaters 105a are disposed on a line type thermal head 105, a shaft 107, which is disposed in the body chassis 101, rotatably supports a head supporting unit 106 which holds the line type thermal head 105, a spring 109 elastically presses the row of heaters 105a onto the platen roller 102 sandwiching recording paper 108 between the row of heaters 105a and the platen roller 102, and a recording paper holder 110 holds the rolled recording paper 108.
The directions of feeding and ejecting the recording paper 108 in the conventional thermal line printer having the above structure is described hereinafter referring to FIG. 36.
As shown in FIG. 36, the recording paper 108 is fed from the short side of the body chassis 101 in a plane projecting the body chassis 101 along the axial direction of the platen roller 102 through the space between the platen roller 102 and the recording paper feeding guide 101a disposed in the body chassis 101 as shown by an arrow A and ejected from the long side of the body chassis 101 in the same projecting plane after passing through a pressed portion between the row of heaters 105a disposed on the line type thermal head 105 and the platen roller 102, or, the recording paper 108 is fed from the long side of the same projecting plane along the axial direction of the platen roller 102 through a space at a recording paper feeding guide (not shown) disposed in the body chassis 101 as shown by an arrow B and ejected from the long side of the body chassis 101 after passing through the pressed portion between the row of heaters 105a disposed on the line type thermal head 105 and the platen roller 102.
Next, the state of installation of a thermal line printer in a handy terminal as an example is described referring to FIG. 37. In FIG. 37, the thermal line printer is illustrated with solid lines for the convenience of showing the layout of the installation of the printer, though the printer is actually contained inside the body of the handy terminal.
In FIG. 37, a thermal line printer is disposed behind rows of operation keys 112, a display unit 113, a control circuit substrate 114 and a battery 115 in the body, 111 of a handy terminal, and a rolled recording paper 108 is disposed at the back end. In the above structure, the recording paper is ejected from the upper side after printed by the thermal line printer, whereby the user can see the state of the printing.
However, under the circumstance that the smaller and thinner type is desired, the conventional thermal line printer having the above structure has been desired to be reduced in the dimension of depth rather than the height since the height (i.e., the dimension of Y in FIG. 36) of the thermal line printer can be reduced as the height is determined by the size of the paper holder for containing necessary length of rolled recording paper. Therefore the reduction of the dimension of depth rather than that of height is strongly desired.
For reducing the dimension of depth, there is a method that the conventional thermal line printer is set upright as shown in FIG. 38 and the paper is fed from the long side of the body chassis 101 in a plane projecting the body chassis 101 along the axial direction of the platen roller 102 through the space at the recording paper feeding guide disposed in the body chassis 101 and ejected from the other long side of the body chassis 101 in the same projecting plane after passing through a pressed portion between the row of heaters 105a disposed on the line type thermal head 105 and the platen roller 102. However, in this method, the ejected paper after printing falls down for the gravity thereof toward the user's side when the printer is installed in a handy terminal or the like as shown in FIG. 39. Therefore the user cannot see the state of printing.
On the other hand, in a conventional driving device for a thermal line printer, dynamically segmenting operation is taken for reducing the size of a power source and for increasing printing speed. In the dynamically segmenting operation, a block to be printed is dynamically varied according to the number of dots to be printed. FIG. 40 shows the general printing process of one dot line by the thermal line printer which executes the dynamically segmenting operation as described above.
In the process, as shown in FIG. 40, the number of dots to be printed in the present dot line is counted at first, and a block to be printed by the thermal line head at one time is determined in such a manner that the number of dots does not exceed a predetermined maximum number of dots printed by simultaneous application of electricity. Then the number of segments of the thermal line head necessary for printing one dot line is determined. Then a pulse width Th applied to the thermal line head is determined based on parameters such as the above number of segments, the temperature of the thermal line head, voltage applied to the thermal line head and the like. Then the rotation cycle period (hereafter, rotation period) of the stepping motor for operation in the present dot line is determined by taking, after comparison, the longer period from the following: a standard motor rotation period stored in advance, and a period computed by multiplying the pulse width Th by the number of segments of the thermal line head. Lastly, the stepping motor is operated with the rotation period determined in the above, also the thermal line head is operated. FIG. 41 shows an example of the timing chart of the above operation.
However, in the above conventional printing method, as shown in FIG. 41, when the pulse width Th applied to the thermal line head is longer, the difference between a motor rotation period in a second dot line (i.e., TM2=Th2.times.6-segment) and a motor rotation period in a third dot line (i.e., TM3=standard motor rotation period) becomes larger. In general, in a stepping motor, when the fluctuation of the rotation periods is larger, the vibration becomes large, whereby the vibration noise becomes larger. Especially, when the rotation period changes suddenly from a long motor period due to the numerous segments of the thermal line head to a short motor period due to the few segments, the step out of the stepping motor is liable to occur.
On the other hand, when the temperature of the thermal line head is low, or when voltage applied is low, or in the case of the numerous segments of the thermal line head, the pulse width Th becomes long. When the temperature is low, load to the mechanism of the thermal line printer becomes large, which causes the step out of the stepping motor. Also when the voltage applied is low, the torque of the stepping motor becomes weak, which also causes the step out of the stepping motor to the level of vital inconvenience in the thermal line printer.
Also, when the standard motor rotation period is set long for decreasing the difference between TM2 and TM3, there has been a problem that the period of "TOFF" shown in FIG. 41 becomes long all the time, which causes the decrease of printing speed. FIG. 42 and FIG. 43 show timing charts in which numeric values are put in for further explanation on the above operation. FIG. 42 shows an example in which a large difference between motor rotation period in a second dot line (7.2 ms) and motor rotation period in a third dot line (3.0 ms) causes a large vibration of the motor, also causes the step out of the motor. In FIG. 43, the standard motor rotation period is set long, whereby the "TOFF" period becomes long, which causes the decrease of printing speed.
In the above description on the prior art, the number of segments of the thermal line head is varied between one and six for the convenience of showing the operation by illustrations. However, the number of segments is varied between one and some hundreds in practical use.